1. Field of the Invention
This invention relates generally to active sonar systems and more particularly to methods for optimizing noise-limited and reverberation-limited target detection in littoral regions.
2. Description of the Prior Art
A major problem for sonar systems operating in shallow water is reverberation from the ocean bottom. With expanding Navy operation in littoral regions, the shallow-water reverberation problem has received much recent attention from practitioners in the art. In 1995, Henry Cox et al. (Cox and Lai, xe2x80x9cGeometric Comb Waveforms for Reverberation Suppression,xe2x80x9d Proceedings, Twenty-Ninth Asilomar Conference on Signals, Systems, and Computers,xe2x80x9d Pacific Grove, Calif., Oct. 29-Nov. 1, 1995, pp. 1185-1189) proposed a class of geometric comb waveforms that offer high range resolution and excellent Doppler properties for active sonar detection of moving targets in reverberation. Until the introduction of the geometric comb waveform, active sonar practitioners were limited to fighting reverberation by using one of two methods: a spectrally-flat wide-band pulse to spread reverberation noise power over the wide pulse band and minimize reverberation power in each range bin, or a long shaded (e.g., Hanning-weighted) continuous-wave (CW) pulse to concentrate reverberation noise power at the zero-Doppler bin and permit detection of non-zero Doppler targets. The flat wide-band pulse approach has limited effectiveness in multipath echo environments such as encountered in littoral regions and the CW shaped-pulse approach achieves Doppler reverberation rejection at the expense of range resolution. The uniform comb waveform is a variation of the wide-band pulse method that uses aplurality of equally-spaced spectral components (CW tone pulses) where the spacing is selected to be large with respect to the target Doppler shifts. Each spectral component provides an echo with properties similar to the wide-band pulse approach but coherent addition of a plurality N of such spectral components provides a processing gain of 10 log N over a single CW pulse. However, the uniform comb signal is disadvantaged by the large peak-to-average power ratio (large dynamic range) of the transmitted signal, which severely limits available average signal power levels needed in noise-limited environments, and by severe range ambiguity resulting from multiple equal amplitude peaks in the autocorrelation function.
The Cox geometric comb waveform solved the range ambiguity problem by using a plurality of non-uniformly-spaced spectral components (CW tone pulses) whose frequencies are spaced according to a geometric progression. While the geometric comb waveform has been welcomed with enthusiasm by active sonar practitioners because of excellent Doppler properties for suppressing reverberation with acceptable range ambiguity, the peak-to-average power problem, while improved by nearly 10 dB over the uniform comb signal, is still disadvantageous in noise-limited littoral regions. Cox et al. suggest easing the problem somewhat by clipping the spectral-component peaks to reduce the requisite transmitter dynamic range, but this introduces spectral distortion that can corrupt other processing gains.
T. Collins et al. (Collins and Atkins, xe2x80x9cDoppler-Sensitive Active Sonar Pulse Designs for Reverberation Processing,xe2x80x9d IEE Proc.-Radar, Sonar Navig., Vol. 145, No. 6, December 1998, pp.347-353) later compare the theoretical and experimental performance of several reverberation-insensitive active sonar waveforms. Collins et al. show that the linear period-modulated (LPM) chirp waveform is best for low Doppler targets at long ranges and the sinusoidally frequency-modulated (SFM) pulse waveform is preferred for suppressing reverberation effects, except that the Cox comb waveform eliminate much of the range-ambiguity of the SFM system.
Many littoral regions have negligible reverberation and detection capability is accordingly ambient-noise limited over some portion of the nominal detection range of an active sonar system. This may occur in slightly deeper water at close range or in shallow water at longer range. Because active sonar transmitters suitable for littoral operation are normally power- and duty-cycle-limited, there is a need for transmit waveforms with dynamic range limited to make use of as much available power as possible. Collins et al. suggest that the SFM waveform is preferred over the Cox comb waveform despite the resulting range-ambiguity problems because of the improved noise-limited performance of the higher average transmitter power available from SFM.
There is accordingly still a clearly-felt need in the art for an active sonar system that provides improved detection performance in either reverberation-limited or noise-limited littoral regions. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the active sonar comb-waveform power-limitation problem by introducing for the first time a system employing a new comb waveform herein denominated the triplet-pair comb waveform. Ambient noise-limited performance of the system of this invention is superior to that of systems employing other Doppler-sensitive waveforms such as the geometric comb waveform. Reverberation-limited performance of the system of this invention is slightly inferior to that of systems employing other Doppler-sensitive waveforms but this invention eliminates much of the range ambiguity problems seen with other non-comb waveforms.
It is a purpose of this invention to provide an active sonar system with improved noise-limited performance in littoral regions with reverberation.
In one aspect, the invention is an acoustic detection method comprising the steps of transmitting an acoustic signal employing a triplet-pair comb waveform to ensonify a target area, detecting acoustic reflections from the target area at a receiver transducer, generating a transducer output signal representing the acoustic reflections, and processing the transducer output signal to determine range and Doppler values for the target area.
In a preferred embodiment, the invention is an acoustic detection apparatus comprising an acoustic transmitter for transmitting an acoustic signal to ensonify a target area, wherein the acoustic signal includes a triplet-pair comb waveform, a receiver transducer for detecting acoustic reflections from the target area, a circuit for generating a transducer output signal representing the acoustic reflections, and a signal processor for processing the transducer output signal to determine range and Doppler values for the target area.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.